Producing corrosion-resistant surfaces on metals



Patented June 5, 1928.

UNITED STATES 1,672,444 PATENT OFFICE.

FREDERICK M. BECKET. OF NEW YORK, N. Y., ASSIGNOR 'IOELECTED-METALLURGI- CAL COMPANY,

A CORPORATION OF WEST VIRGINIA.

PRODUCING CORROSION-RESISTANT SURFACES ON METALS.

No Drawing.

The invent-ion is a process by cementation alloyage acorrosion-resistant surface on an article composed of a corrodible metalor alloy, and the invention also includes articles made by such aprocess.

The term corrosion-resisting as used herein refers to the ability toresist the attack of corrosive agents whether normally gaseous, liquidor solid. Examples of gaseous corrosive agents are oxygen-containingmixtures and products of combustion. These often carry water vapor orsuspended particles of liquid or solid corrosive substances whic renderthem even more destructive. The commonest liquid corrosive agents aresolutions of reactive chemical compounds such as acids. The absorptionof carbon by metals is for the purposes of this application to beconsidered as a type of corrosion. Iron and steel have but slightresistance against the attacks of any of these classes of corrosiveagents yet they have such outstanding advantages in other directionsthat their use is far more wide-spread than that of any other metals. Aprimary object of the invention is to render iron and steel resistant tocorrosion by providing them with a protective casing of the kindreferred to, and especially to treat iron and steel so as to render themresistant to the action of oxidizing gases at elevated temperatures.

Cementation processes wherein a chemical element is caused to metal attemperatures below the melting points of the metal and the diffusingelement have been studied. The processes oommercially known assherardizing and calorizing are of this type, zinc and aluminum beingthe metals which are respectively caused to diffuse in these processes.It has also been proposed to pack iron articles in powdered chromium andthen to apply heat to'cause the chromium to form a superficial layer ofcementation alloy with the iron.

r In the process last referred to, which is usually called chromizing,the temperature required is above 1300 C. and is so high that thechromium, presenting a large surface because of its powdered condition,becomes superficially oxidized in any usual atmosphere, such as onecontaining products of combustion, and thus loses in a'large measure itspower to dilfuse into the metal to be cased. For this reason it isnecessary to maintain an atmosphere of inert gas such for producingdiffuse into a mass of I Application filed March 17, 1926. Serial No.95,470.

as hydrogen about the powdered chromium, and this again makes itpractically imperative to employ electrical heating.

I have discovered that the presence of a content of silicon in thecementation mixture will considerably depress the temperature at whichthe diffusion takes place, and that the temperature can be so loweredwhen operating in this wa that the chromium is not detrimentally oxiized by such gas mixtures as air or products of combustion. It may bethat the presence of silicon directly dih minishes the oxidizability ofthe chromium to some degree, but however this may be the net result isthat an inert atmosphere becomes unnecessary. Heating by the combustionof gas or other fuel thus becomes practicable. Electrical heating can ofcourse be used if preferred and an inert atmosphere can be maintained.

The silicon is preferably introduced into the material as an alloy, andit is advantageously alloyed with the chromium contained in the mixture.A ternary alloy of iron, chromium, and silicon possesses certainadvantages as the cementation material. It is cheaper than the binaryalloy of silicon and chromium and also appears to act more rapidly andeffectively. An important advantage over pocesses using powderedchromium follows the use of silicon alloys since the latter are brittleand easily powdered while the making of chromium powder is quiredifficult.

The cementation mixture preferably contains in addition to the activeagent or agents a diluent, such as an inert oxide. Excellent resultshave been obtained with alumina, which has previously been used inprocesses of this type, but various other oxides and inert powderedmaterials can be used. A principal purpose of the diluent in the presentprocess is to restrict the rate at which the cementation material entersthe surface of the article to be cased so that this rate does not toogreatly exceed the rate at which the material penetrates toward theinterior of the article. Otherwise there may be too much accumulation ofsilicon at the surface of the article, producing an alloy of low meltingpoint which will undergo incipient fusion, especially at edges. Thepro-- decreased and it becomes necessary to use higher temperatures orlonger times to obtain a case of the desired properties. With increasing, silicon there is an increased tendency for an effect alreadyreferred to, viz fusion at the surface of the article under treatment.

Temperatures above 900 C. may be empl0yed' and it is usually unnecessaryto raise the temperature above 1200 C. Between 1100 C. and 1150 C. thealloyage proceeds rapidly but at an easily controllable rate,

and this temperature range is preferred. At the temperatures suitablefor use silicon has a stronger tendency to diffuse into steel than doeschormium. Therefore, if a cementation mixture is used which contains asmuch silicon as chormium the penetrated portion of the steel willcontain more silicon than chromium. For some purposes this isundesirable. For example when the aritcle is being treated to adapt itto resist oxidation at high temperatures a high chromium content in thecoating is desirable. This can be brought about by increasing thechromium: silicon ratio in the cementation material. Thus, when a steelarticle was treated at 1100 C. with a ferrochrome silicon containingapproximately 40% each of silicon and chromium. there was five or sixtimes as much silicon as chromium in the surface layer of the treatedarticle. With a ferrochrome silicon containing about 8% of silicon andabout of chromium, these two elements diffused in nearly equalproportions, but a temperature of 1175 C. was needed to bring about anacceptable rate of action. A'protective layer in which the chromium:silicon ratio was sufliciently high was obtained with ferrochromesilicone containing 15% to 20% of silicon and about 55% chromium, and inthis case a temperature of 1130 C. was sufficient.

The cementat-ion mixtures may be, used in; definitely if they areperiodically replenished in the constituents which have been removedfrom them. In many instances a casing about 0.03 inches in thickness isdesired and this may usually be produced in from three to ten hours. Thesize of the article treated is not materially changed during theprocess. There is nothing critical about the degree of fineness in whichthe materials of the cementation mixture are applied. Material whichwill pass a 30 mesh s1eve or even coarser material may be used though inmost of my experiments the materialhas been crushed andground to pass a60 mesh sieve.

The invention is by no means restricted to the introduction of chromiuminto the article treated as numerous other metals may be introducedsimultaneously or instead of the chromium. The latter metal excels inits ability to diminish oxidation at elevated temperatures and it isthereforeof special importance. Silicon alloys are in generalresistant-to acids and other chemicals and a sufficient silicon contentgenerally diminishes the tendenc of a metal to take up carbon. Theseeffects are added to the effect of the chromium in reducing oxidation orto the specific effect of any other metal used with or instead ofchromium.

Where it is desired to augment the resist ance to acid attack.carbonization or other special form of corrosion, other elements knownto be effective for such purposes when alloyed with silicon, for examplemanganese, nickel, zirconium. titanium and tungsten, may be introduced.the chromium being retained or not depending on the particular purposein view. The silicon facilitates the diffusion of these other metals inmuch the same Way that it does with chromium, in addition tocontributing its own characteristic effect to the superficial alloy.

The casings or coatings formed the invention are remarkably adherent.When subjected to deformation sufficiently severe to.crack them, thecracks are normal to the surface and there is but little tendency forthe case -to flake ofi'.

I claim:

1. Process of forming on a ferrous article amined composition containingchromium and silicon, which comprises applying to the article an excessof powdered ferrochrome-silicon having a higher chromium-to-silicon.

ratio than is to be present in the case, and thereafter maintaining thearticle in contact with the ferrochrome silicon at a cementingtemperature until the article is effectively cased.

. 2. Process of forming on a ferrous article a corrosion-resistingintegral case of predetermined composition containing chromium andsilicon, which comprises applying to the article an excess of powderedmaterial having a higher chromium-to-silicon ratio than is to be presentin the case, and thereafter maintaining the article in contact with saidmaterial at a cementing temperature until the article is effectivelycased.

In testimony whereof, I aflix my signature.

FREDERICK M. BECKET.

